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Aug 17, 2015 - ABSTRACT: We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior ...
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The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior Ran Zhang,†,§ Shuanhong Ma,†,§ Qiangbing Wei,† Qian Ye,† Bo Yu,*,† Jasper van der Gucht,*,‡ and Feng Zhou*,† †

State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Middle Rd, 730000 Lanzhou, China ‡ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, NL-6703 HB Wageningen, The Netherlands § University of Chinese Academy of Sciences, 100049 Beijing, China S Supporting Information *

ABSTRACT: We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brushcovered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect of surfactant on lubrication.

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INTRODUCTION Human joints show an ultralow friction coefficient (0.001− 0.01) for typical pressures of ∼5 MPa at human hips or knees, which is mainly attributed to the adaptability of elastic articular cartilage and the brush-like structure of water-soluble macromolecules in synovial fluids.1,2 Surface grafted polymer brushes in aqueous media can simulate the lubrication of synovial joints by taking advantage of the ultralow friction coefficient and good biocompatibility of polymer brushes. It has been previously observed that neutral or charged polymer brushes can serve as good boundary lubricants due to the combination of high hydration of the polymer brushes leading to resistance to compression and a fluid-like response to shear.3−10 Their frictional behavior is influenced by various factors, such as monomer structure, polymer architecture, chain length, grafting density, solvent quality, and charges on the polymer chains.11,12 Besides the excellent water lubrication performance, polymer brushes also provide an ideal platform for building intelligent surfaces because of their inherent responsive behavior to external stimuli, which can induce a swelling−collapse transition of the grafted chains, resulting in dynamic changes in interfacial properties, such as wettability,13,14 anti-icing,15 and lubricating properties.16 Previous work has mainly focused on © XXXX American Chemical Society

conformational changes (and resulting changes in wettability and friction) in response to electrochemical potential,17 pH,18 solvent,19,20 and counterions.13,15,21−23 Based on the responsive smart polymeric surfaces, ion-pairing interaction is a common and versatile approach to achieve tunable wettability and friction. Recent work in our group has shown that the friction of polyelectrolyte brush covered surfaces can be tuned from superior lubrication (μ ∼ 10−3) to ultrahigh friction (μ > 1) via exchanging counterions.16 The interaction between polymers and surfactants is an interesting phenomenon that has been widely investigated for many years, both experimentally and theoretically,14,24−27 because of the wide range of applications in cosmetics, paints, detergents, foods, and formulations of drugs and pesticides.28 Cohen Stuart et al.24 have investigated the adsorption of anionic surfactants in poly(ethylene oxide) (PEO) brushes and found that the number of adsorbed surfactant molecules per PEO monomer decreases rapidly with increasing brush density due to excluded volume interactions and electrostatic repulsion Received: June 11, 2015 Revised: August 13, 2015

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Macromolecules Scheme 1. Schematic Diagram of the Weak Interaction between Surfactants and Polymer Brushes on Si Substrates

and oligo (ethylene glycol) methacrylates (OEGMA, average Mn 360, J&K) were used as received. 2,2′-Bipyridyl (bipy, AR), sodium dodecyl sulfate (SDS, AR), sodium laurate (SL, AR), and hexadecyltrimethylammonium bromide (CTAB, AR) were commercially available and used without any purification. Copper(I) bromide was purified via reflux in acetic acid. Other general reagents and solvents were used as received. Silane initiator 3-(trichlorosilyl)propyl 2-bromo-2-methylpropanoate and thiol initiator ω-mercaptoundecyl bromoisobutyrate were synthesized according to previous reports.32,33 Preparation of Polymer Brushes. Polymer brushes were prepared by versatile surface-initiated atom transfer radical polymerization (SI-ATRP).34 Initiator-modified Si substrates were prepared by vapor deposition of silane initiator under vacuum. The general polymerization procedure was as follows: the monomer was dissolved in a mixture of methanol/water under N2 flow for 15 min. Then, bipyridyl and CuBr were added to the Schlenk tube and purged with N2 flow again; the mixture was stirred under N2 for 15 min until a homogeneous dark-brown solution formed. In the final, initiatormodified substrates were put into a Schlenk tube with N2 protection. After polymerization for 1 h, the samples were taken out and rinsed with pure water and methanol and then dried under a flow of nitrogen. Polymerization recipes for the four monomers are as follows: METAC (80% in water) 8 mL, bipy 160 mg, CuBr 80 mg, 8 mL of water/ methanol (v/v = 2:1); SPMA 6 g, bipy 80 mg, CuBr 35 mg, 12 mL of water/methanol (v/v = 2:1); OEGMA 1.8 g, bipy 41.5 mg, CuBr 16.5 mg, 4 mL of water and 1 mL of methanol; SBMA 4 g, bipy 120 mg, CuBr 40 mg, and 8 mL of water/methanol (v/v = 2:1). Exchange of surfactants was performed by immersing brushes modified wafers into a solution of target surfactants for 30 min. The wafers were taken out and immersed in pure water for 30 min and then followed by drying under a flow of nitrogen. The in situ friction tests were performed by injecting 40 μL surfactant solution on the interface between PDMS ball and brushes modified wafer. Characterization. Static contact angle (CA) was acquired using a DSA-100 optical contact angle meter (Kruss Company, Ltd., Germany) at ambient temperature (25 °C). A droplet of 5 μL of deionized water was used as probe liquid. The average contact angle values were obtained by measuring the sample at three different positions on the substrate. The thickness of the polymer layer was measured using a spectroscopic ellipsometer (Gaertner model L116E) equipped with a He−Ne laser source (λ = 632.8 nm) at a fixed angle of incidence of 50°. The refractive index of the polymer film was 1.5. Surface topography was obtained using an atomic force microscope (AFM) (Agilent 5500) in tapping mode using a commercially available

between the negatively charged surfactant micelles. Konradi and Ruhe25 have studied the interaction of poly(methacrylic acid) (PMAA) brushes with oppositely charged surfactants as a function of the polymer brush grafting density, the surfactant concentration, and the surfactant alkyl chain length. The PMAA brush shrinks very strongly at a surfactant concentration of around 10−5 mol/L, despite the fact that the surfactant uptake is relatively low. Moreover, the surfactants can act as very efficient boundary lubricants due to the strong repulsion between their charged and hydrated headgroups.29,30 Dedinaite et al.31 have explored surface interactions and friction forces acting between layers composed of mixtures of cationic polyelectrolytes and anionic surfactants on negatively charged surfaces using the AFM-colloidal probe technique. They found that mixed polyelectrolyte−surfactant layers exhibit low friction forces and a high load bearing capacity. In this paper, we systematically study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brushes. Below the CMC of the surfactant, exchange of the counterions in the brush with oppositely charged surfactants (Scheme 1) leads to a strong increase of the friction coefficient, from ∼10−2 to nearly ∼100. For negatively charged poly(3sulfopropyl methacrylate potassium salt) (PSPMA) brushes with cationic surfactants (CTAB), we observe a reduction in friction after the CMC, but not for positively charged poly[2(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes with anionic SDS surfactants. This suggests a different interaction mechanism between polymer and surfactant in these two cases. For zwitterionic and neutral polymer brushes, we observe very little influence of either cationic or anionic surfactants, indicating that electrostatic interactions between polymer and surfactant play an important role.

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EXPERIMENTAL SECTION

Materials and Chemicals. 3-Sulfopropyl methacrylate potassium salt (SPMA, 95%, TCI), 2-(methacryloyloxy)ethyltrimethylammonium chloride (METAC, 80% in water, TCI), [2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (SBMA, 97%, Aldrich), B

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Figure 1. (A) Changes in friction coefficients and wettability (inserted images) for a PSPMA brush on Si substrates after exchanging with different concentration of CTAB. (B) Changes in friction coefficients for bare silicon surfaces after in situ injection of different concentrations of CTAB. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N.

Figure 2. (A, B) Changes in friction coefficients corresponding to PSPMA brush on Si substrates after exchanging with 0.5 mmol/L CTAB for different time and friction curve vs time after in situ injection of 40 μL of 0.5 mmol/L CTAB. (C, D) Changes in friction coefficients corresponding to PSPMA brush on Si substrates after exchanging with 5 mmol/L CTAB for different time and friction curve vs time after in situ injection of 40 μL of 5 mM CTAB. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N. Friction Test. The macroscopic friction properties during aqueouslubricated sliding have been characterized by means of conventional pin-on-disk reciprocating tribometry (UMT-2, CETR) by recording the friction coefficient (μ) at different sliding conditions. Elastomeric poly(dimethylsiloxane) (PDMS) hemisphere with a diameter of 5 mm was employed as a pin against polymer brushes in water. A commercial silicone elastomer kit (SYLGARD 184 silicone elastomer, base and curing agents, Dow Corning, Midland, MI) was purchased to prepare PDMS pins. For the preparation of hemispherical PDMS pin, a polystyrene 96-well cell culture plate with round shaped well (Dow

type II MAC levers with a normal force constant of 2.8 N/m. Quartz crystal microbalance with dissipation measurements (QCM-D) was performed using a Q-Sense microbalance (Sweden) at 25 °C. Commercially available (QSX-301, QSense) gold-coated quartz chips were used. Chemical composition information about the samples were obtained by X-ray photoelectron spectroscopy (XPS), and the measurement was carried out on an ESCALAB 250xi spectrometer (Thermon Scientific, USA) by using Al Kα radiation. The binding energy of C 1s (284.8 eV) was used as the reference. C

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Figure 3. (A) Changes in friction coefficients for PSPMA brushes on Si substrates after exchanging with 0.1 mmol/L CTAB for different frequencies. (B) Changes in friction coefficients for PSPMA brushes on Si substrates after exchanging with 10 mmol/L CTAB for different frequencies. The friction test was carried out by sliding a silicone elastomer ball under loading of 0.5 N.

Figure 4. Changes in resonance frequency and corresponding dissipation of QCM chip modified with PSPMA brushes exchanged with (A) 0.1 mmol/L solutions of CTAB and (B) 10 mmol/L solutions of CTAB. Corning) was used as a mold. The base and curing agents of SYLGARD 184 elastomer kit were mixed at a 10:1 ratio by weight. After removing bubbles, the mixtures were transferred into the mold under vacuum and then incubated in an oven (70 °C) overnight. The water-contact angle for the PDMS was 110 °C, and the elasticity modulus and Poisson ratio were 2 MPa and 0.5, respectively.35 At least three friction tests were repeated for each sample to get average value. Each friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N. The contact area between the brush-covered substrate and the PDMS sphere was determined by optical microscopy, as described in the Supporting Information. We found a contact area of 1.54 mm2, which is in good agreement with predictions of Hertz contact mechanics, indicating that the contact area is determined mainly by the elastic deformation of the PDMS sphere.

relatively high applied normal forces. In our experiment, a PSPMA brush of about 80 nm thick was prepared on Si by ATRP. This PSPMA brush with K+ counterions exhibited excellent water affinity (CA ∼ 10°) and lubrication (μ ∼ 0.01). After exchanging with different concentration of cationic surfactant, the friction coefficient of the brush increases gradually from 0.01 to 0.8, as shown in Figure 1A. After the CMC (0.9 mmol/L), the friction coefficient decreases again to 0.27. We attribute this change in friction to binding of surfactant molecules to the polymer chains, probably as a result of both electrostatic and hydrophobic interactions. Adsorption of oppositely charged surfactants reduces the net charge of the brush and effectively makes the brush more hydrophobic. This is also clear from the increase in contact angle from 10° to 90°, as seen in the insets of Figure 1A. As a consequence, the brush expels hydration water and collapses, leading to a strong increase in friction.12,23 The maximum friction is observed at a surfactant concentration of around 0.95 mmol/L, which is slightly above the CMC of CTAB. The decrease in friction after the CMC might be attributed to the adsorption of CTAB micelles onto the polymer chains in the brush due to hydrophobic interactions.38 This may lead to reversal of the net charge of the brush. Electrostatic repulsion between the micelles would then lead to swelling of the brush, increasing the

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RESULTS AND DISCUSSION Effects of Cationic Surfactants Concentration on the Water Lubrication of Anionic Polyelectrolyte. Polyanionic brush PSPMA is a strong polyelectrolyte, containing sulfonate groups in the side chains. Brushes of PSPMA revealed extremely low friction coefficients of around 0.01 as a result of the strong hydration of the brushes and the electrostatic repulsion between the charged brushes.36,37 This allows the brushes to maintain a large amount of fluidizing water even at D

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Figure 5. AFM characterization of PSPMA brushes in pure water on gold substrates (A) and after exchange with 0.1 mM CTAB (B) and 10 mM CTAB (C): (a) AFM height images of patterned brushes. (b) AFM images of PSPMA brushes; the image size is 4 μm × 4 μm. (c) Cross-section analysis of patterned brushes.

Table 1. XPS Analysis Results of PSPMA and after Exchanging with Surfactant Samples atom concentration (%) samples

C

O

S

K

N

PSPMA 0.1 mM CTAB 1 mM CTAB 10 mM CTAB

62.19 74.49 75.58 76.87

27.55 17.24 16.5 15.26

6.33 4.17 3.3 3.47

3.92 0.86 1.3 0.76

0 3.24 3.33 3.64

concentration, the friction coefficient increased from 0.6 to 0.8, and the presence of micelles also reduced the friction coefficient of silicon. The exchange velocity of ions in the PSPMA brush depends on the surfactant concentration.39 We chose two different concentrations of CTAB to investigate the exchange velocity (Figure 2). For 0.5 mmol/L CTAB (Figure 2A), a stable friction coefficient (∼0.2) was reached after 10 min, indicating that the surfactant adsorption was completed within 10 min. When we performed the friction test via in situ injection of 40 μL of 0.5 mM CTAB (Figure 2B), after a certain time the

Figure 6. XPS spectra of PSPMA brushes (a) and after exchanging with 0.1 mM CTAB (b), 1 mM CTAB (c), and 10 mM CTAB (d).

water content and thereby the lubrication properties, leading to a reduced friction.12 Indeed, the contact angle decreases slightly after the CMC. Figure 1B shows changes in the friction coefficient for bare silicon surfaces after in situ injection of different concentrations of CTAB. With increasing surfactant E

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Figure 7. Changes in friction coefficients and wettability (inserted images) corresponding to PMETAC brushes on Si substrates after exchanging with different concentration of anionic surfactants SDS (A) and SL (B). The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N.

Figure 8. (A, B) Changes in friction coefficients corresponding to PMETAC brushes on Si substrates after exchanging with 0.1 mmol/L SDS for different time and friction curve after injection of 0.1 mmol/L SDS. (C) The friction test of PMETAC brushes after exchanging with 1 mmol/L SDS. (D) The friction test of PMETAC brushes after exchanging with 20 mmol/L SDS. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N.

properties of PSPMA brushes after exchange with different concentrations of CTAB. Figure 3A shows friction coefficients of PSPMA brushes after exchanging with 0.1 mmol/L CTAB at different frequencies; the results show that the friction coefficient does not depend on frequency (or sliding speed) at this surfactant concentration. By contrast, for PSPMA brushes exchanged with 10 mmol/L CTAB solutions, the friction coefficient decreases strongly as the frequency increases (Figure 3B). This reduction in the friction coefficient with velocity could be caused by a transition of friction mode, from boundary lubrication to mixed lubrication,35,41,42 according to the Stribeck curve.43,44 QCM-D measurements are carried out in situ to monitor the physical properties of PSPMA brushes exchanging with

friction coefficient reached a stable value (∼0.12). The value is lower than the original friction coefficient (∼0.2), which is probably because the surfactant molecules on the surface of the polymer brushes formed a boundary lubricant film and showed well lubricating performance.40 For a surfactant concentration of 5 mmol/L CTAB, the friction coefficient reached a stable value (∼0.26) after 2 min, indicating that the exchange rate is faster at this higher concentration. Figure 2D shows the friction coefficient obtained from in situ injection of 40 μL of 5 mM CTAB during the measurement. The stable friction coefficient is 0.05, which is again lower than the original value (∼0.26), probably due to micelles on the surface of the polymer brush that showed excellent lubrication performance. We have also investigated the effect of the frequency on the lubricating F

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Figure 9. Changes in resonance frequency and corresponding dissipation of QCM chip modified with PMETAC brushes exchanged with (A) 0.1 mM solutions of SDS and (B) 20 mM solutions of SDS.

Figure 10. AFM characterization of PMETAC brushes on gold substrates (A) and after exchanging with 1 mM SDS (B) and 20 mM SDS (C): (a) AFM height images of patterned brushes. (b) AFM images of PMETAC brushes; the image size is 4 μm × 4 μm. (c) Cross-section analysis of patterned brushes.

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electroneutral. The dramatic decrease of the dissipation indicates that the brush chains partially collapsed, and the surface became rigid due to the exclusion of water. At a higher surfactant concentration of 10 mmol/L CTAB (Figure 4B), the surfactant adsorption seems to proceed in two phases: an initial rapid decrease in resonance frequency (probably associated with the fast adsorption of surfactant molecules), followed by a slower, more gradual decrease in resonance frequency (probably due to the slower binding of micelles). Upon rinsing, only the latter contribution disappears, while the initially adsorbed surfactants remain. This suggests that in the second stage the net charge of the layer is reversed. We further characterized the surface topography of polymer brushes after exchange with surfactant using AFM. As shown in Figure 5, the PSPMA brushes with K+ cations swelled to ∼120 nm in pure water while it has 50 nm “dry” thickness at room temperature. However, the PSPMA brushes swelled to ∼150 nm in pure water when exchanged with 0.1 mmol/L CTAB. Together with the QCM measurement, it can be speculated that PSPMA takes up hydrated surfactant molecules, which does not result in exclusion of water from the brush interior, but a net mass increase. For 10 mmol/L CTAB exchanged PSPMA brush, it swelled to ∼160 nm in pure water due to the repulsion between the internal micelles; however, the micelles are too small to be observed on the surface of brushes. Moreover, the rms roughnesses of PSPMA brushes are 1.08 nm and then increased to 1.76 nm, 2.49 nm after exchange with 0.1 and 10 mM CTAB, respectively, which is due to dehydration of the brushes after exchange. XPS is a powerful tool to clarify the composition and chemical state of the elements on the polymer surfaces. Surface XPS analysis of PSPMA brushes (Figure 6 and Table 1) proved that the surfactant exchange has occurred. The strong S 2p signal at 168.7 eV was detected on Si substrate covered with a PSPMA brush (Figure 6a), proving the presence of polymer on the substrate. The N 1s signal appeared after exchange with CTAB, while the K 2p signal decreased, proving the exchange of K+ for CTAB. Effects of Anionic Surfactants on the Water Lubrication of Cationic Polyelectrolyte. Polyelectrolyte chains will present a strongly stretched conformation in pure water due to the electrostatic repulsion between neighboring charged groups. 46 Surfaces covered with a brush of poly[2(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) (about 40 nm in thickness) with Cl− counterions exhibited extremely low friction coefficient in water (μ ∼ 0.01) and excellent water affinity (CA ∼ 10°), indicating strong swelling of these brushes in water. Here, the anionic surfactants sodium dodecyl sulfate (SDS) and sodium laurate (SL) are selected to exchange with Cl−. The effects on the friction coefficients and wettability are summarized in Figure 7. Similarly as for the PSPMA brushes with CTAB, the surfactant exchange led to a strong increase of the friction coefficient, from 0.01 to 1 (Figure 7A), which illustrates that PMETAC brushes showed a strong deswelling upon interaction with SDS solutions even at very low surfactant concentrations. The exchange also led the contact angle of the surface to increase from 10° to 80°. For SL exchanged PMETAC brushes, the friction coefficient increased gradually from 0.01 to 0.10 and the contact angle of the surface increased to 60° with increasing concentration. Different from PSPMA system, we do not observe a maximum in the friction coefficient for the PMETAC brushes, but the friction coefficient remains more or less

Figure 11. XPS spectra of PMETAC brushes (a) and after exchanging with 1 mM SDS (b), 10 mM SDS (c), and 20 mM SDS (d).

Table 2. XPS Analysis Results of PMETAC and after Exchanging with Surfactants atom concentration (%) samples

C

O

N

Cl

S

PMETAC 1 mM SDS 10 mM SDS 20 mM SDS

72.77 71.94 72.82 66.23

20.95 20.73 19.19 14.24

5.15 3.51 3.87 10.46

1.13 0.45 0.37 0.31

0 3.37 3.75 8.77

Figure 12. Friction coefficients of neutral POEGMA brushes under water after exchanging with different anionic (SDS) and cationic (CTAB) surfactants. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N.

surfactants. A change in resonance frequency (Δf) is related to the mass of the adsorbed film on a quartz chip. Dissipation (defined as ΔD = Edissipated/2πEstored) represents the capacity of brushes to release mechanical energy and gives information on the rigidity of the film.45 For PSPMA brushes, after exchange with 0.1 mmol/L CTAB solution, the decrease in resonance frequency was attributed to the substitution of K+ by heavier CTAB molecules (Figure 4A). During the exchange process of CTAB, the resonance frequency slightly declines, meaning that part of the surfactant molecules continuously adsorbed in the polymer brushes or at their surface. After rinsing with water, the surface does not return to its original state, indicating that the adsorption is irreversible. The surfactant does not desorb from the brush in pure water because the brush has to remain H

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Figure 13. Changes in resonance frequency of QCM chip modified with POEGMA brushes exchanged with different anionic (SDS) and cationic (CTAB) surfactants.

coefficient declined quickly and then remained stable after exchanging for a while. Probably, the surfactant molecules on the surface of the polymer lead to a good lubricating performance.40 After in situ exchanging with 20 mmol/L SDS (Figure 8D), a stable friction coefficient (∼0.05) can be gained throughout the whole process, which is far below the original value (∼1.11), indicating that a large number of micelles absorbed on the surface of the polymer brush, leading to excellent lubrication properties. The kinetics of surfactant adsorption and resulting rigidity changes of PMETAC brushes were monitored using QCM-D. Figure 9 shows the changes in resonance frequency and dissipation of a PMETAC brush coated quartz chip during exchange of Cl− counterions with 0.1 and 20 mM solutions of SDS, respectively. First, a rapid increase in resonance frequency was observed after exchanging with 0.1 mmol/L SDS. This indicates that the total mass of the brush based chip decreases, even though the molecular weight of SDS (FW: 288.4) is higher than that of Cl− counterions (FW: 35.5). We conclude that the binding of surfactant leads to a strong exclusion of water and a collapse of the brush, also explaining the high friction coefficient. The decrease in dissipation indicates an increase of the rigidity of the surface, which is also caused by the collapse of the brush. Again, the surfactants do not desorb upon flushing with water because they are tightly bound by electrostatic forces. At 20 mM SDS, similar behavior is found (Figure 9B). Here, the resonance frequency increases even further upon rinsing with water, probably as a result of desorption of micelles. The surface topography of PMETAC brushes was probed by AFM as shown in Figure 10. The height of the polymer patterns was 80 nm in pure water and increased to 90 and 100

Figure 14. Friction coefficients of zwitterionic PSBMA brushes under water after exchanging with different anionic (SDS) and cationic (CTAB) surfactants. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N.

constant after the CMC. Therefore, we speculate that there are no micelles adsorbed on the surface of PMETAC brushes or inside the brush after exchanging, probably because the PMETAC chains are slightly less hydrophobic than the PSPMA chains (which have one additional carbon atom in the side chain), leading to a weaker binding. The lubrication mechanism below and above the CMC are therefore similar. We further investigated the exchange velocity of PMETAC brushes in surfactant solution (Figure 8). For 0.1 mmol/L SDS (Figure 8A), a stable friction coefficient (∼0.9) was reached after 5 min. In an in situ friction test (Figure 8B), the friction coefficient became stable (∼0.78) after exchanging for a longer time. Figure 8C shows the friction test in situ of PMETAC brush after exchanging with 1 mmol/L SDS. The friction I

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Figure 15. Changes in resonance frequency and corresponding dissipation of QCM chip modified with PSBMA brushes exchanged with (A) 1 mM solutions of SDS and (B) 20 mM solutions of SDS.

was almost no change in contact angle and friction coefficient (Figure 14), nor did the “dry” thickness change. Figure 15 shows the QCM-D study of PSBMA brushes during exchange with different concentrations of SDS. We find a decrease in resonance frequency after immersing in a 1 mM SDS solution, which indicates the adsorption of surfactant molecules in the brush (Figure 15A). Contrary to our findings for charged brushes, we here find that the resonance frequency returns to its original value after rinsing with water, indicating that the absorbed surfactant molecules are easily washed away. Electrostatic interactions are much weaker in this case, and surfactants can leave the brush without losing electroneutrality. At higher surfactant concentrations, similar findings were obtained (Figure 15B), but with a faster kinetics.

nm after exchange with 1 and 20 mM SDS. Mass increase may indicate that the uptake of surfactant molecules was counterbalanced by exclusion of water from brush interior. As shown in Figure 10b, the surface roughness increases significantly with added surfactant, also implying strong ion pairing interaction between surfactant and polymer brushes. Moreover, we do not observe the existence of micelles on the surface of the brush, probably due to very weak adhesion of micelles to the polymer brush surface; this is in agreement with our earlier friction tests. Surface XPS analysis of PMETAC brushes (Figure 11 and Table 2) revealed the surface compositions. The S 2p signal at 168.7 eV that appears after surfactant exchange can be attributed to the presence of SDS. Moreover, the intensity of S 2p signal became stronger with exchange concentration increasing. Effects of Anionic and Cationic Surfactants on the Water Lubrication of Neutral Polymer Brushes. Good lubrication properties can also be obtained with hydrophilic neutral polymer brushes that swell in water,47 such as poly(ethylene oxide) (PEO) brushes, although the efficiency of lubrication is reported to be less than that of charged (or zwitterionic) brushes.40 Here, we prepared POEGMA brushes (about 60 nm thick on silicon wafer), which showed a very low friction coefficient (μ ∼ 0.02) and strong hydrophilicity (CA ∼ 30°). Contrary to what we observed for the charged brushes, no significant change in friction coefficient is observed upon exchanging with cationic or anionic surfactant for the POEGMA brush (Figure 12). QCM-D measurements also indicate that there is very little effect of surfactant on the properties of the brush (Figure 13). Hence, ionic surfactants do not interact with hydrophilic POEGMA brushes. Effects of Anionic and Cationic Surfactants on the Water Lubrication of Polyzwitterionic Brushes. Polyzwitterionic brushes contain both negatively and positively charged groups; they have been reported to have very good lubrication properties as a result of the high level of hydration.11,48 Here, we prepare zwitterionic PSBMA brushes of ∼20 nm by ATRP. The PSBMA chains contain quaternary ammonium groups and sulfonate groups fixed on the same chain to maintain a net neutral state.40 We find that the PSBMA brushes exhibit very good lubrication ability (μ ∼ 0.015). After exchange with different anionic and cationic surfactants, there

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CONCLUSIONS

The weak interaction between polymer brushes and oppositely charged surfactants has been systematically studied, which is mainly governed by electrostatic interaction and hydrophobic interaction. Our experiments show that the lubricating behavior and wettability of polymer brushes depend on the surfactant concentration. Below the CMC, adsorption of surfactant molecules in the polyelectrolyte brush increases the hydrophobicity of the brush and thereby leads to dehydration and an increase of the friction coefficient. For cationic and anionic brushes, the weak interaction achieved a gradual transition from ultralow friction to ultrahigh friction. Above the CMC, for negatively charged PSPMA brushes we observe a reduction in friction due to the lubricating effect of micelles, while for positively charged PMETAC brushes we find a much higher friction without a lubricating effect of the micelles. This suggests a different interaction mechanism between polymer and surfactant in these two cases. For zwitterionic and neutral brushes, for which electrostatic and hydrophobic interactions could be negligible, there is nearly no uptake of surfactants, so that neither anionic nor cationic surfactants have any influence on the lubricating properties. Our findings thus present important clues to uncovering the relation between surfactant adsorption and lubrication behavior. J

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01267. Determination of real contact area; comparison to Hertz contact mechanics; Figure S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (B.Y.). *E-mail [email protected] (F.Z.). *E-mail [email protected] (J.v.d.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (21434009, 21204095, 21125316) and Key Research Program of CAS (KJZD-EW-M01).

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REFERENCES

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DOI: 10.1021/acs.macromol.5b01267 Macromolecules XXXX, XXX, XXX−XXX